"resembling the great Delta of the Greeks"

Delta is the fourth letter of the Greek alphabet (Δ).  Its triangular shape is evident at the mouth of large rivers, such as the Nile, where the slow moving water spreads across the plain, depositing sediment before flowing into the sea.  The Greek’s called this area of a river the delta, after its shape. 

Nile delta
Public DomainNile delta - Credit: NASA